A piezoelectric resonator unit such as a quartz crystal resonator unit comprising a piezoelectric body such as a quartz crystal element having an electrode formed on the surface thereof is popularly used in an oscillating circuit for generating a prescribed frequency. Recently, there is an increasing demand for a piezoelectric resonator unit giving stable characteristics at a high accuracy as an oscillation source of a reference signal for communication equipment. FIG. 11 illustrates a front view (FIG. 11(a)) and a side view (FIG. 11(b)) of a schematic configuration of a typical conventional piezoelectric resonator unit. A piezoelectric resonator 10 of the piezoelectric resonator unit 8 is provided with a resonator element 5 comprising a thin and flat quartz crystal piece 1 as a piezoelectric body, having an electrode 3 formed on each of the two surfaces thereof by vapor deposition or the like, and a cylindrical plug 11 supporting the resonator element 5. The plug 11 has a metal frame 13 surrounding an insulating member 12 made of a material such as covar glass having an electrolytically plated outer periphery, and two round bar-shaped leads 15 running through this insulating member 12. Leading ends 16 of these two leads 15 are respectively solder-connected to connecting electrodes 4 of the resonator element 5 so as to be conductive with the exterior of the plug 11 serving as a supporting member via the leads 15. The leads 15 serve also to mount the resonator element 5 onto the plug 11. Further, the piezoelectric resonator 10 shown in FIG. 11 mounts the resonator element 5 by holding it between the leads 15 to form a supporting structure with a very high rigidity.
Main steps for assembling a piezoelectric resonator unit by the use of such a piezoelectric resonator 10 are illustrated in FIG. 12. After the completion of Step 21 of mounting the resonator 5 onto the plug 11 serving as the supporting member as described above, a final resonance frequency adjustment is carried out in Step 22 by adjusting the film thickness of the electrode 3 of the resonator element 5 through vapor deposition or sputtering. Then, in Step 23, the piezoelectric resonator 10 is inserted and sealed into a case 9 in vacuum or in an inert gas atmosphere to assemble the piezoelectric resonator unit 8. Then, after conducting tests of frequency, CI-value and temperature characteristics in Step 24, the product is shipped.
As described above, in addition to the demand for a piezoelectric resonator or a piezoelectric oscillator comprising a combination of a piezoelectric resonator unit and a semiconductor as a clock source of a CPU and the like, a demand is also recently increasing for a reference signal source of communication equipment, and it is becoming increasingly necessary to have a high accuracy and stable characteristics more than the conventional reference signal source of communication equipment. Particularly, room-temperature aging property is required to be improved from about ±3 to 5 ppm/year to about ±1 ppm/year, and further, there is a demand for a piezoelectric resonator unit or a piezoelectric oscillator, in consideration of the application in a portable equipment, excellent in impact resistance and high in durability.
In the conventional piezoelectric resonator unit shown in FIG. 11, in which the leads are connected to the electrode by the use of solder 17, the connecting step exhibits a satisfactory operability, and a high connecting strength is available. When left at a high temperature of 80 to 125° C., however, solder may diffuse into the electrode, this resulting in easier deterioration of aging property and in easier occurrence of fluctuation of frequency. In addition, mounting by the use of soldering requires a preheating step at about 150 to 250° C., although for a short period of time, and a heating step at about 350° C. As a result, in a certain state of heating, there may be a distortion of temperature characteristics. When the resonator element is AT-cut, deviation occurs from an ideal cubic curve, and this makes it difficult to obtain a highly accurate frequency even by performing temperature correction. Such a phenomenon is believed to be attributable to the fact that local heating of the piezoelectric body upon connection causes a difference in temperature, crystals having different properties such as β-quartz crystal having no piezoelectric phenomenon are produced in the piezoelectric body, though to a minimum extent.
Furthermore, the conventional piezoelectric resonator unit shown in FIG. 11, in which the resonator element is connected to the plug in a state with a very high rigidity, is highly resistant to micro-vibration. The unit however has a poor resistant to a strong impact such as dropping, thus resulting in breakage or peeling of the resonator element, and hence making the unit unserviceable at a high probability.
To overcome these defects, Japanese Unexamined Patent Publication No. 6-303,077 discloses a technique of connecting the leads and the resonator element by means of a conductive adhesive in place of solder, and connecting the leads only to a side surface of the resonator element. Although there is not an explicit description in the Japanese Unexamined Patent Publication No. 6-303,077, with the use of such a conductive adhesive, solder diffusion can be avoided even when left exposed to a high temperature as described above, thus permitting improvement of aging property, and absence of heating to a high temperature upon mounting leads to a slight distortion of temperature characteristics, thus making it possible to provide a product of a very high accuracy. In this technique, flattened leads are attached to one side surface of the resonator element, in place of mounting the resonator element by holding it between high-rigidity bar-shaped leads. It is therefore known that a high impact resistance is available, and the probability of becoming unserviceable as a result of dropping (dropping property) becomes lower. Even in a resonator element adopting such a supporting method, a piezoelectric resonator unit 8 using a piezoelectric body 2 of a convex shape as shown in FIG. 13 results in a relatively large weight of the resonator element 5, and stress tends to be concentrated at the constriction at the leading end of the mounting section, thus causing an occurrence of peeling. It is therefore impossible to achieve a remarkable improvement in the dropping property.
Further, in a piezoelectric resonator 10 shown in FIG. 13, the leading end 16 of the bar-shaped lead 15 is flattened into substantially a U shape, and the leading end 16 and a connecting electrode 4 are connected with a conductive adhesive 19. A plug-side edge 5a of the resonator element 5 is mounted onto the plug 11 with a non-conductive adhesive 18. There is therefore available an improved operability as compared with the conventional piezoelectric resonator 10 shown in FIG. 11, in which mounting is accomplished so that both sides of the resonator element are in contact with the bar-shaped leads. However, in view of the time and labor required for injecting the conductive adhesive into an appropriate gap formed between the leading end 16 and the connecting electrode 4, and for holding the resonator element 5 and the plug 11 at prescribed positions with the use of devices and jigs until setting of the injected conductive adhesive, operability of the step of causing the coated adhesive to set is not high as compared with the step for carrying out soldering, and the devices and jigs for positioning the resonator element and the plug have a low operating efficiency.
An object of the present invention is therefore to provide a piezoelectric resonator and a piezoelectric resonator unit, in which a resonator element is mounted on a supporting member by means of a resin adhesive, excellent in aging property and high in accuracy, wherein impact resistance is further improved. Another object of the invention is to provide a piezoelectric resonator and a method for manufacturing the same, which permits the efficient manufacture of a piezoelectric resonator and a piezoelectric resonator unit in which mounting is accomplished by the use of such a resin. A further object of the invention is to provide at a low cost a piezoelectric resonator and a piezoelectric resonator unit for a communication equipment of a high demand by providing a piezoelectric resonator and a piezoelectric resonator unit high in productivity and accuracy.